An heir apparent or heiress apparent is a person who is first in a line of succession and cannot be displaced from inheriting by the birth of another person. An heir presumptive, by contrast, is someone who is first in line to inherit a title but who can be displaced by the birth of a more eligible heir.

In a hereditary system governed by some form of primogeniture, an heir apparent is easily identifiable as the person whose position as first in the line of succession to a title or office is secure, regardless of future births. An heir presumptive, by contrast, can always be "bumped down" in the succession by the birth of somebody more closely related in a legal sense (according to that form of primogeniture) to the current title-holder.

The clearest example occurs in the case of a holder of a hereditary title, one that can only be inherited by a single person, with no children. If at any time he were to produce children, they (the offspring of the title-holder) rank ahead of whatever more "distant" relative (the title-holder's sibling, perhaps, or a nephew or cousin) had been heir presumptive.

Many legal systems assume childbirth is always possible regardless of age or health. In such circumstances a person may be, in a practical sense, the heir apparent but still, legally speaking, heir presumptive. Indeed, when Queen Victoria succeeded her uncle King William IV, the wording of the proclamation even gave as a caveat:

...saving the rights of any issue of his late Majesty King William IV, which may be born of his late Majesty's consort.

This provided for the possibility that William's wife, Adelaide of Saxe-Meiningen, was pregnant at the moment of his death, since such a posthumous child, regardless of its sex, would have displaced Victoria from the throne.[2] Adelaide was 44 at the time, so pregnancy was possible even if unlikely.

Daughters (and their lines) may inherit titles that descend according to male-preference primogeniture, but only in default of sons (and their heirs). That is, both female and male offspring have the right to a place somewhere in the order of succession, but when it comes to what that place is, a female will rank behind her brothers regardless of their ages or her age.

Thus, normally, even an only daughter will not be heir apparent, since at any time a brother might be born who, though younger, would assume that position. Hence, she is an heir presumptive. For example, Queen Elizabeth II was heir presumptive during the reign of her father, King George VI, because at any stage up to his death, George could have fathered a legitimate son.

In a system of absolute primogeniture that disregards gender, female heirs apparent occur. As succession to titles, positions, or offices in the past most often favoured males than females, females considered to be an heir apparent were rare. Absolute primogeniture was not practised by any modern monarchy for succession to their thrones until the late twentieth century with Sweden being the first to adopt absolute primogeniture in 1980 and other Western European monarchies following suit.

But even in legal systems that apply male-preference primogeniture, female heirs apparent are by no means impossible: if a male heir apparent dies leaving no sons but at least one daughter, then the eldest daughter would replace her father as heir apparent to whatever throne or title is concerned, but only when it has become clear that the widow of the deceased is not pregnant. Then, as the representative of her father's line she would assume a place ahead of any more distant relatives. Such a situation has not to date occurred with the English or British throne; several times an heir apparent has died, but each example has either been childless or left a son or sons. However, there have been several female heirs apparent to British peerages (e.g. Frances Ward, 6th Baroness Dudley, and Henrietta Wentworth, 6th Baroness Wentworth).

In one special case, however, England and Scotland had a female heir apparent. The Revolutionsettlement that established William and Mary as joint monarchs in 1689 only gave the power to continue the succession through issue to Mary II, eldest daughter of the previous king, James II. William, by contrast, was to reign for life only, and his (hypothetical) children by a wife other than Mary would be placed in his original place (as Mary's first cousin) in the line of succession – after Mary's younger sister Anne. Thus, although after Mary's death William continued to reign, he had no power to beget direct heirs,[3] and Anne became the heir apparent for the remainder of William's reign. She eventually succeeded him as Queen of England, Scotland and Ireland.

Parliament deposed James Francis Edward Stuart, the infant son of King James II & VII (of England and Scotland respectively) whom James II was raising as a Catholic, as the King's legal heir apparent—declaring that James had, de facto, abdicated— and offered the throne to James II's oldest daughter, the young prince's much older Protestant half-sister, Mary (along with her husband, Prince William of Orange). When the exiled King James died in 1701, his Jacobite supporters proclaimed the exiled Prince James Francis Edward as King James III of England and James VIII of Scotland; but neither he nor his descendants were ever successful in their bids for the throne.

Crown Prince Gustav (later known as Gustav, Prince of Vasa), son of Gustav IV Adolf of Sweden, lost his place when his father was deposed and replaced by Gustav IV Adolf's aged uncle, the Duke Carl, who became Charles XIII of Sweden in 1809. The aged King Charles XIII did not have surviving sons, and Prince Gustav was the only living male of the whole dynasty (besides his deposed father), but the prince was never regarded as heir of Charles XIII, although there were factions in the Riksdag and elsewhere in Sweden who desired to preserve him, and, in the subsequent constitutional elections, supported his election as his grand-uncle's successor. Instead, the government proceeded to have a new crown prince elected (which was the proper constitutional action, if no male heir was left in the dynasty), and the Riksdag elected first August, Prince of Augustenborg, and then, after August's death, the Prince of Ponte Corvo (Marshal Jean-Baptiste Bernadotte, who acceded as Charles XIV John in 1818). The two lines united later, when Charles XIV John's great-grandson Crown Prince Gustaf (who acceded as Gustaf V in 1907) married Gustav IV Adolf's great-granddaughter Victoria of Baden, who became Crown Princess of Sweden. Thus, from Gustav VI Adolf onwards, the kings of Sweden are direct descendants of both Gustav IV Adolf and his son's replacement as crown prince, Charles XIV John.

Prince Carl Philip of Sweden, at his birth in 1979, was heir apparent to the throne of Sweden. Less than eight months later, a change in that country's succession laws instituted absolute primogeniture, and Carl Philip was supplanted as heir apparent by his elder sister Victoria.

In some jurisdictions, an heir apparent can automatically lose that status by breaching certain constitutional rules. Today, for example:

A British heir apparent would lose this status if he became a Catholic. This is the only religion-based restriction on the heir-apparent. Previously, marrying a Catholic also equated to losing this status, however, in October 2011, the governments of the 16 Commonwealth realms —of which Queen Elizabeth II is monarch— agreed to remove the restriction on marriage to a Catholic. All of the Commonwealth realms subsequently passed legislation to implement the change, which fully took effect in March 2015.

Swedish Crown Princes and Crown Princesses would lose heir apparent status, according to the Act of Succession, if they married without approval of the monarch and the Government, abandoned the "pure Evangelical faith", or accepted another throne without the approval of the Riksdag.

Dutch Princes and Princesses of Orange would lose status as heir to the throne if they married without the approval of the States-General, or simply renounced the right.

Spanish Princes and Princesses of Asturias would lose status if they married against the express prohibition of the monarch or the Cortes.

Belgian Dukes and Duchesses of Brabant would lose heir apparent status if they married without the consent of the monarch, or became monarch of another country.

Danish Crown Princes and Princesses would lose status if they married without the permission of the monarch. When the monarch grants permission for a dynast to enter marriage, he may set conditions that must be met for the dynasts and/or their children to gain or maintain a place in the line of succession; this also applies for Crown Princes and Princesses.

Monarchy of Luxembourg

The Grand Duke of Luxembourg is the monarchical head of state of Luxembourg. Luxembourg has been a grand duchy since 15 March 1815, when it was elevated from a duchy, and was in personal union with the United Kingdom of the Netherlands until 1890 under the House of Orange-Nassau and is the world's only sovereign grand duchy. Since 1815, there have been nine monarchs of Luxembourg, including the incumbent, Henri.

Crown prince

A crown prince is the male heir apparent to the throne in a royal or imperial monarchy. Its female form is crown princess, which may refer either to an heir apparent or, especially in earlier times, the wife of the person styled crown prince.

Line of succession to the Belgian throne

There are fifteen persons in the line of succession to the Belgian throne.

Succession to the Danish throne

The Danish Act of Succession, adopted on 27 March 1953, restricts the throne to those descended from Christian X and his wife, Alexandrine of Mecklenburg-Schwerin, through approved marriages. Succession is governed by absolute primogeniture.

Succession to the Dutch throne

Since 1983, the crown of the Netherlands passes according to absolute primogeniture. From 1814 until 1887, a monarch could only be succeeded by their closest female relative if there were no eligible male relatives. Male-preference cognatic primogeniture was adopted in 1887, though abolished when absolute primogeniture was introduced in 1983. Proximity of blood has been taken into consideration since 1922, when the constitution was changed to limit the line of succession to three degrees of kinship from the current monarch.

Succession to the Swedish throne

The line of succession to the Swedish throne is determined by the Act of Succession, originally approved jointly by the Riksdag of the Estates assembled in Örebro and King Charles XIII in 1810.

Succession to the British throne

Succession to the British throne is determined by descent, sex, legitimacy, and religion. Under common law, the Crown is inherited by a sovereign's children or by a childless sovereign's nearest collateral line. The Bill of Rights 1689 and the Act of Settlement 1701 restrict succession to the throne to the legitimate Protestant descendants of Sophia of Hanover who are in "communion with the Church of England". Spouses of Roman Catholics were disqualified from 1689 until the law was amended in 2015. Protestant descendants of those excluded for being Roman Catholics are eligible.

Danish royal family

The Danish royal family is the dynastic family of the monarch. All members of the Danish royal family except Queen Margrethe II hold the title of Prince/Princess of Denmark. Dynastic children of the monarch and of the heir apparent are accorded the style of His/Her Royal Highness, while other members of the dynasty are addressed as His/Her Highness. The Queen is styled Her Majesty.

Line of succession to the former Greek throne

The Greek monarchy was abolished by the then-ruling military regime on 1 June 1973, an act that was confirmed by plebiscite on 8 December 1974 after the regime's fall. The title of king is used by the last reigning monarch, Constantine II. His son, the Crown Prince Pavlos, who was born in 1967, is the heir apparent to the pretended title.

Line of succession to the Bhutanese throne

The line of succession to the throne of Bhutan is based on the constitution of Bhutan. Currently the line of succession is according to male-preference cognatic primogeniture with males preceding females who are in the same degree of kinship. If the heir apparent has reached the age of majority of 21, the monarch would step down at age 65. If the heir apparent and the nearest people in the line of succession are deemed unsuitable, it is up to the monarch to decide who will be the next heir. If the monarch violates the constitution, he or she must abdicate.

Monarchy of Denmark

The Monarchy of Denmark, colloquially known as the Danish Monarchy, is a constitutional institution and a historic office of the Kingdom of Denmark. The Kingdom includes not only Denmark, but the autonomous regions of Greenland and the Faroe Islands. The Kingdom of Denmark were already consolidated in the 8th century, whose rulers are consistently referred to in Frankish sources as Kings (Reges). Under the rule of King Gudfred in 804 the Kingdom may have included all the major provinces of medieval Denmark
The current unified kingdom of Denmark was founded by the Viking kings Gorm the Old and Harald Bluetooth in the 10th century, making the monarchy of Denmark the oldest in Europe. Originally an elective monarchy, it became hereditary only in the 17th century during the reign of Frederick III. A decisive transition to a constitutional monarchy occurred in 1849 with the writing of the first Constitution. The current Royal House is a branch of the princely family of Glücksburg, originally from Schleswig-Holstein in modern-day Germany, the same royal house as the Norwegian and former Greek royal families.

Crown Prince of Thailand

The Crown Prince of Thailand is a title held by the heir apparent to the Thai throne. First created by King Chulalongkorn in 1886, for his son Prince Maha Vajirunhis, the king's eldest son by a royal wife Queen Savang Vadhana. Prior to this, the Siamese throne did not have a law or formal system regulating the royal succession. In 1688 King Petracha of Ayutthaya created the title of Front Palace, which by the Rattanakosin period had become the main title granted to the heir presumptive to the throne. However few Front Palaces have succeeded to the throne this way, with the exception of King Buddha Loetla Nabhalai in 1809. After the Front Palace crisis in 1875, the title of Front Palace was abolished and replaced with the title of Crown Prince, who became heir apparent to the throne.